Force sensing sheet

ABSTRACT

A force sensing array includes multiple layers of material that are arranged to define an elastically stretchable sensing sheet. The sensing sheet may be placed underneath a patient to detect interface forces or pressures between the patient and the support structure that the patient is positioned on. The force sensing array includes a plurality of force sensors. The force sensors are defined where a row conductor and a column conductor approach each other on opposite sides of a force sensing material, such as a piezoresistive material. In order to reduce electrical cross talk between the plurality of sensors, a semiconductive material is included adjacent the force sensing material to create a PN junction with the force sensing material. This PN junction acts as a diode, limiting current flow to essentially one direction, which, in turn, reduces cross talk between the multiple sensors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 13/453,461 filed Apr. 23, 2012 by GeoffreyTaylor and entitled ELASTICALLY STRETCHABLE FABRIC FORCE SENSOR ARRAYSAND METHODS OF MAKING; which in turn is a continuation of U.S. patentapplication Ser. No. 12/380,845 filed Mar. 5, 2009 by Geoffrey Taylor,and entitled ELASTICALLY STRETCHABLE FABRIC FORCE SENSOR ARRAYS ANDMETHOD OF MAKING.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates to transducers or sensors used to measureforces or pressures exerted on a surface.

B. Description of Background Art

Whenever a human body is supported by an object such as a chair or bed,normal and shear forces produced in reaction to the weight of theindividual are transmitted from the supporting surface through the skin,adipose tissues, muscles, etc. to the skeleton. The forces exerted onbody parts by support surfaces, which are equal and opposite to bodyweight forces, can in some cases cause damage to tissues. Forces on bodyparts can compress internal blood vessels and occlude nutrients from thetissue, the product of the magnitude and duration of these forcesdetermining whether tissue damage or morbidity will occur. The areas ofthe human body which are most at risk of developing tissue damage suchas a pressure sore are: heel, ischial tuberosities, greater trochanter,occiput and sacrum.

Some prior art sensor arrays for sensing patient pressure have sufferedfrom disadvantages. For example, with some prior art sensors arrays, ifthe array is used to measure pressures exerted on a human body by a veryform-fitting, conformal wheelchair seat cushion or extremely lowpressure bed mattress or cushion, the array will often interfere withthe function of the cushion or bed support surface, and give erroneousforce measurements which are used to map the way the bed or chairsupports a person. Such errors result from a “hammocking” effect, inwhich a flexible but not drapable sensor array deployed between fixedsupport positions cannot conform precisely to the shape of a patient.This effect can occur for example, using sensor arrays that use wirecore sensing elements which make the arrays essentially non-stretchable.The lack of conformability of a sensor array alters the way a cushion orbed supports a patient, and also frequently results in forces orpressures exerted on individual sensors in the array being larger than apatient would actually encounter in the absence of the sensor array.

Another situation in which existing force sensor arrays for measuringand mapping forces exerted on human body parts are less thansatisfactory occurs when attempting to make such measurements in anon-obtrusive, non-interfering manner on body parts which have complexshapes such as the feet.

Still further, in some prior art sensor arrays, it can be difficult tomeasure the resistance of sensor elements in an array using matrixaddressing of the sensor elements. The difficulty results from the factthat the electrical resistances of all the non-addressed sensor elementsin an array shunts the resistance of each addressed sensor element,resulting in cross-talk inaccuracies in measurements of individualsensor element resistances.

SUMMARY OF THE INVENTION

Briefly stated, the present invention comprehends novel pressure orforce sensing transducers which include individual force sensingelements that are arranged in a planar array on or within a substrateconsisting of a thin, flexible polymer sheet or a thin sheet of woven ornon-woven fabric.

According to one embodiment of the invention, a flexible force sensingarray is provided that includes an elastically stretchable sheet, aplurality of first conductive paths, a layer of sensing material, alayer of semiconductive material, and a plurality of second conductivepaths. The first conductive paths are supported on the elasticallystretchable sheet. The layer of sensing material is positioned incontact with the first conductive paths and the layer of sensingmaterial has an electrical characteristic that varies in response tophysical forces exerted on it. The layer of semiconductive material ispositioned in contact with the layer of sensing material on a side ofthe layer of sensing material opposite the plurality of first conductivepaths. The plurality of second conductive paths are positioned incontact with the layer of semiconductive material on a side of the layerof semiconductive material opposite the layer of sensing material.

According to another embodiment, a flexible force sensing array isprovided that includes a first elastically stretchable sheet, aplurality of first conductive paths, an intermediate elasticallystretchable sheet, a layer of semiconductive material, a secondelastically stretchable sheet, and a plurality of second conductivepaths. The plurality of first conductive paths are supported on thefirst elastically stretchable sheet and are parallel to each other. Theintermediate elastically stretchable sheet is positioned in contact withthe first elastically stretchable sheet and includes sensing materialthereon that has an electrical characteristic that varies in response toapplied physical forces. The layer of semiconductive material ispositioned in contact with the intermediate elastically stretchablesheet to thereby form with the sensing material a PN junction. Theplurality of second conductive paths are supported on the secondelastically stretchable sheet and are in electrical contact with thesemiconductive material. The second conductive paths are parallel toeach other and transverse to the first conductive paths.

According to other embodiments, the sensing material is a piezoresistivematerial. The piezoresistive material may be supported by an elasticallystretchable substrate. The elastically stretchable substrate may be madeat least partially of nylon. The first and second elasticallystretchable sheets may both be made from woven fabric. The woven fabricis nylon in one embodiment.

The semiconductive layer may be coated onto the layer of sensingmaterial and include a metallic oxide. In some embodiments, the metallicoxide may include copper oxide.

A cover sheet may be included in some embodiments that is made from anelastically stretchable material. In some embodiments, the cover is apolyurethane or polyvinyl chloride.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partly broken away perspective view of a basic embodiment ofa three-layer piezoresistive thread pressure sensor array according toone embodiment, which uses a pair of polymer film outer substrates and acentral piezoresistive layer.

FIG. 2 is a vertical transverse sectional view or end view of the sensorarray of FIG. 1 taken in the direction 2-2.

FIG. 3 is a partly broken-away, upper perspective view of a second,two-layer embodiment of a piezoresistive thread pressure sensor array,in which the central piezoresistive layer shown in the basic embodimentof FIGS. 1 and 2 is replaced by a piezoresistive coating on conductivethreads of the sensor array.

FIG. 4 is a vertical transverse sectional or end view of the sensorarray of FIG. 3, taken in the direction 4-4.

FIG. 5 is a fragmentary perspective view of a modification of the sensorarray of FIGS. 1 and 3 in which adjacent pairs of more closely packedrow and column conductor threads are spatially and electrically isolatedfrom each other by non-conductive threads.

FIG. 6A is a fragmentary transverse sectional view of the sensor arrayof FIGS. 1 and 2, on a further enlarged scale, showing the dispositionof crossed row and column conductive threads contacting a centralpiezoresistive layer to form force sensing elements, with no externalforce applied to the elements.

FIG. 6B is a view similar to that of FIG. 6A, but with a moderate normalforce applied to the sensor elements.

FIG. 6C shows the sensor elements with a larger external force appliedthereto.

FIG. 7 is a graph showing electrical resistance plotted as a function offorce or pressure exerted on sensor elements of the sensor arrays shownin FIGS. 1 and 3.

FIG. 8A is a fragmentary transverse sectional view of the sensor arrayof FIGS. 3 and 4 on a further enlarged scale, showing the disposition ofrow and column piezoresistive threads to form force sensing elements,with no external force applied to the array.

FIG. 8B is a view similar to that of FIG. 8A, but with a moderate normalforce applied to the sensor elements.

FIG. 8C shows the sensor element with a larger external force appliedthereto.

FIG. 9 is a partly broken-away perspective view of a three-layerembodiment of a piezoresistive threads pressure sensor array, which usesa pair of fabric outer substrates and a central piezoresistive layer.

FIG. 10 is a fragmentary view of the sensor array of FIG. 9 on anenlarged scale and showing a lower plan view of an upper horizontal rowconductor part of the sensor array.

FIG. 11 is a fragmentary view of the sensor array of FIG. 9, on anenlarged scale and showing an upper plan view of a lower vertical columnconductor part of the sensor array.

FIG. 12 is a vertical transverse sectional view, of the sensor array ofFIG. 9, taken in the direction 12-12.

FIG. 13A is a partly broken-away, exploded upper perspective view of afourth, two-layer piezoresistive thread pressure sensor array usingfabric substrates in which the central piezoresistive layer of theembodiment shown in FIG. 9 is replaced by a piezoresistive coating onconductive threads of the sensor array.

FIG. 13B is a vertical transverse sectional view of the sensor array ofFIG. 13A, taken in the direction 13B-13B.

FIG. 14 is a partly broken-away upper perspective view of a fifth,single layer embodiment of a piezoresistive thread pressure sensor arraywhich has a single fabric substrate, in which both row and columnpiezoresistive threads are fastened to the same side of a singleinsulating substrate sheet.

FIG. 15 is an upper plan view of the sensor array of FIG. 14.

FIG. 16 is a vertical transverse sectional view of the sensor array ofFIG. 14, taken in the direction 16-16.

FIG. 17 is partly broken-away, exploded upper perspective view of amodification of the fabric substrate sensor arrays of FIG. 9, 13 or 14in which lower column conductive threads of the sensor array aredisposed in a sinuous arrangement on the fabric lower substrate panel.

FIG. 18 is an upper perspective view of another modification of thesingle layer fabric substrate sensor array of FIG. 14 in which both therow and column conductive threads are sinuously arranged and located onopposite sides of a piezoresistive substrate sheet.

FIG. 19 is an upper plan view of the sensor array of FIG. 18.

FIG. 20 is a lower plan view of the sensor array of FIG. 18.

FIG. 21 is a vertical transverse sectional view of the sensor array ofFIG. 19.

FIG. 21A is a fragmentary upper perspective view of a single layerfabric substrate sensor array in which both upper row and lower columnpiezoresistive threads are sinuously arranged and fastened to the sameside of a single insulating substrate sheet.

FIG. 22A is a schematic diagram showing the number of conductivelead-outs required to measure the resistance of individual sensorelements in a linear array.

FIG. 22B shows sensor elements which do not have to be in a lineararrangement.

FIG. 23 is a schematic diagram showing a reduced number of lead-outs formatrix addressing an array of sensor elements arranged in a matrixarray, including, but not limited to, the sensor array of FIG. 39.

FIG. 24 is a schematic diagram showing sensor elements of the array ofFIG. 23 modified to include a diode junction.

FIG. 25 is an upper perspective view of a force measuring sensorapparatus using two-layer sensor arrays of the type shown in FIG. 5.

FIG. 26 is a block diagram showing the sensor array of FIGS. 1 and 3interconnected with signal processing and display circuitry to comprisea force measurement system.

FIG. 27A is a perspective view of a sock incorporating the sensory arrayof FIG. 14-16 or 17-20.

FIG. 27B is a horizontal transverse sectional view of the sock of FIG.27A.

FIG. 28 is a typical electrical resistance-versus-normal force diagramof the sensors disclosed herein.

FIG. 29 is a partly schematic view of modifications of sensor elementsof the arrays of FIG. 1 and FIG. 35, in which sensor elements of thearray have been modified to provide them with P-N, diode-type junctions.

FIG. 30 is a current-versus-voltage diagram for the sensor elements ofFIG. 27 and.

FIG. 31 is an exploded perspective view of another embodiment of a forcesensor array.

FIG. 32 is a perspective view of the sensor array of FIG. 31.

FIG. 33 is an exploded perspective view of components of anotherembodiment of a force sensor array.

FIG. 34 is a perspective view of the sensor array of FIG. 33.

FIG. 35 is a partly diagrammatic perspective view of a body supportcushion apparatus with adaptive body force concentration minimizationaccording to the present intention.

FIG. 36A is a fragmentary upper perspective view of the apparatus ofFIG. 35, showing a sensor array jacket of the apparatus removed from amattress overlay cushion of the apparatus to thereby reveal individualair bladder cells of the mattress.

FIG. 36B is a fragmentary view of the mattress overlay of FIG. 36A,showing an individual air cell thereof.

FIG. 37 is a diagrammatic side elevation view of the apparatus of FIGS.35 and 36, showing certain bladder cells thereof deflated to reducesupport forces exerted on parts of a human body supported by themattress overlay.

FIG. 38 is a vertical sectional view of the mattress of FIG. 36, takenin the direction of line 4-4.

FIG. 39 is a fragmentary exploded perspective view of the mattress ofFIG. 35, showing elements of a force sensor arrangement thereof.

FIG. 40 is a diagrammatic view showing an exemplary relationship betweenthe dimensions of adjacent air bladder cells and the width of aninsulating strip between conductors of sensors on the cells.

FIG. 41 is a block diagram of electro-pneumatic controller elements ofthe apparatus of FIG. 35.

FIG. 42 is a simplified perspective view of the electro-pneumaticcontroller of FIG. 41.

FIG. 43 is a flow chart showing operation of the apparatus of FIG. 35.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIGS. 1-43 illustrate various aspects of elastically stretchable,conformable fabric force sensor arrays, and methods for making thearrays, according to the present invention.

Referring first to FIGS. 1 and 2, a first, basic, three-layer embodimentof a force sensor array is shown.

As shown in FIGS. 1 and 2, a three-layer force sensor array 30 includesa plurality m of elongated, straight thin conductive row threads 31-1through 31-m and a plurality n of elongated, straight thin, conductivecolumn threads 32-1 through 32-n.

The electrically conductive row threads 31 and column threads 32 consistof an elastically stretchable monofilament or woven polymer core 31C,32C, which has been treated to make the threads electrically conductive,as by silver plating the core to form coatings 31P, 32P on cores 31C,32C, respectively.

One type of example embodiment of a sensor array 30 used row and columnconductive threads 31, 32 made from silver plated nylon thread, 117/17 2ply, catalog #A264, obtained from LESS EMF, 809 Madison Avenue, Albany,N.Y. 12208, USA. That conductive thread had a lineal resistivity ofabout 75 ohms per foot, and an elastic stretchability of about 1percent, i.e., at least 10 times greater than that of a stainless steelwire of a similar diameter.

A second type of example embodiment of a sensor array uses row andcolumn conductive threads made from silver plated stretchy nylon yarn,that plated yarn having the designation Shieldex, ®Lycra dtex 20,obtained from W. Zimmerman, GmbH & Co. K6, Riederstrasse 7, D-88171,Weiter-Simmerberg, Germany. That conductive thread had a linealresistivity of about 500 ohms per foot. The elastic stretchability ofthat conductive yarn is greater than 30 percent, i.e., at least 300times greater than that of a stainless steel wire of a similar diameter.

As shown in FIGS. 1 and 2, a row threads 31 and column threads 32 lie inparallel planes but are inclined with respect to one another, such as atan angle of ninety-degrees. In the example embodiment 30, row conductivethreads 31 are fastened to the lower surface 34 on an upper substratesheet 33, and column conductive threads 32 are fastened to the uppersurface 36 of a lower substrate sheet 35.

As may be seen best by referring to FIG. 2, sensor array 30 includes athin central lamination or sheet 37 made of a piezoresistive material.As shown in FIG. 2, opposed inner facing outer surfaces 38, 39 of rowand column conductive threads tangentially contact upper and lowersurfaces 40, 41, respectively, of central piezoresistive sheet 37. Thus,as shown in FIGS. 1 and 2, each crossing point or intersection of a rowconductive thread 31 and a column conductive thread 32 forms apiezoresistive sensor element 48 which consists of a small portion ofcentral piezoresistive sheet 37 that is electrically conductivelycontacted by a row conductive thread and a column conductive thread.

In example embodiments of sensor array 32, piezoresistive sheet 37 wasfabricated by coating a stretchy, i.e., elastically stretchable thinLycra-like fabric sheet with a piezoresistive material. A suitablefabric sheet, which forms a matrix for supporting the piezoresistivematerial, was a fabric known by the trade name Platinum, Milliken, Style#247579, obtained from the manufacturer, Milliken & Company,Spartenburg, S.C., USA. That fabric had a fiber content of 69 percentnylon and 31 percent Spandex, a thread count of about 88 threads perinch, and a thickness of 0.010 inch.

The piezoresistive material used to coat the fabric matrix is made asfollows:

A solution of graphite, carbon powder, nickel powder and acrylic binderare mixed in proportions as required to obtain the desired resistanceand piezoresistive properties. Silver coated nickel flake is used toachieve force response in the low force range of 0 to 1 psi, graphite isused for the mid range of 1 to 5 psi and Charcoal Lamp Black is used forhigh force range of 5 to 1000 psi. Following is a description of thesubstances which are constituents of the piezoresistive material:

Silver Coated Nickel Flake:

-   -   Platelets approximately one micron thick and 5 microns in        diameter.    -   Screen Analysis (−325 Mesh) 95%.    -   Apparent Density 2.8.    -   Microtrac d50/microns 12-17.    -   Available from: Novamet Specialty Products Corporation,        -   681 Lawlins Road, Wyckoff, N.J. 07481

Graphite Powder:

-   -   Synthetic graphite, AC-4722T    -   Available from: Anachemia Science        -   4-214 De Baets Street        -   Winnipeg, MB R2J 3W6

Charcoal Lamp Black Powder:

-   -   Anachemia Part number AC-2155    -   Available from: Anachemia Science        -   4-214 De Baets Street        -   Winnipeg, MB R2J 3W6

Acrylic Binder:

-   -   Staticide Acrylic High Performance Floor Finish    -   P/N 4000-1 Ph 8.4 to 9.0    -   Available from: Static Specialties Co. Ltd.        -   1371-4 Church Street        -   Bohemia, N.Y. 11716

Following are examples of mixtures used to make piezoresistive materialshaving different sensitivities:

Example I for forces in the range of 0 to 30 psi:

-   -   200 ml of acrylic binder    -   10 ml of nickel flake powder    -   10 ml of graphite powder    -   20 ml of carbon black

Example II for forces in the range of 0-100 psi

-   -   200 ml of acrylic binder    -   5 ml of nickel flake powder    -   5 ml of graphite powder    -   30 ml of carbon black

Example III for forces in the range of 0-1000 psi

-   -   200 ml of acrylic binder    -   1 ml of nickel flake powder    -   1 ml of graphite powder    -   40 ml of carbon black

The fabric matrix for piezoresistive sheet 37 is submerged in thepiezoresistive coating mixture. Excess material is rolled off and thesheet is hung and allowed to air dry.

Upper and lower substrate sheets 33, 34 are made of a thin, flexibleinsulating material, such as 0.002 inch thick polyurethane or polyvinylchloride (PVC). In one embodimenty, the substrate sheets 33, 34 are madeof an elastomeric material which has a relatively high degree of elasticstretchability, so that sensor array 30 is readily stretchable andconformable to the surface of an irregularly-shaped object. It can beappreciated, however, that conductive threads 31, 32 should also beelastically stretchable to facilitate stretchability of sensor array 30.This is because conductive threads 31, 32 are affixed to substrate sheet33, 34, respectively, by, for example, blobs of adhesive 42, as shown inFIG. 2. Piezoresistive sheet 37 is also fixed to upper and lowersubstrate sheets 33, 34 by blobs of glue 42.

FIGS. 6A-6C illustrate how the arrangement of row and column conductivethreads 31, 32, in combination with central piezoresistive layer 37 ofsensor array 30 shown in FIGS. 1 and 2, form individual force sensingelements 48. Each force sensor element 48 is located at the cross-overor intersection point 49 of a row conductive thread, e.g., 31-1, 31-2, .. . 31-m, with a column conductive thread, e.g., 32-1, 32-2, . . . 32-n,for a MXN matrix of sensor elements. Thus, individual sensor elementsmay be identified by the nomenclature 48-XXX-YYY, where XXX denotes rownumber and YYY denotes column number.

As shown in FIGS. 2 and 6A, with no external force applied to sensorarray 30, at each cross-over point 49 of a row conductive thread 31 anda column conductive thread 32 of sensor array 30, there is an upperelectrically conductive tangential contact region 43 between centralpiezoresistive layer 37 and the upper conductive row thread, and a lowerelectrically conductive tangential contact region 44 between thepiezoresistive layer and the lower, column conductive thread.

With no external force applied to sensor array 30, the electricalresistance between a row conductive thread 31 and column conductivethread 32, which consists of the series resistance of upper contactregion 43, lower contact region 44, and the effective resistance ofpiezoresistive material 45 of piezoresistive layer 37 between the upperand lower contact regions is relatively high. The relatively highresistance results from the fact that in this case, tangential contactregions 43 and 44 are relatively small, and the thickness ofuncompressed piezoresistive volume 45 is at its maximum value. However,as shown in FIGS. 6B and 6C, when sensor array 30 is placed on asupporting surface S and a normal force N of increasing magnitude isapplied to upper surface 47 of the sensor array 30, the electricalresistance between a row conductive thread 31 and a column conductivethread 32 decreases, as will now be described.

Referring still to FIGS. 2 and 6A, it may be seen that with no externalforce applied to sensor array 30, tangential contact regions 43, 44between row and column conductive threads 31, 32 and centralpiezoresistive layer 37 are relatively small, since the threads have acircular outer cross-sectional shape, which tangentially contacts flatplanar surfaces of the piezoresistive layer. Under these circumstances,the small sizes of contact regions 43, 44 results in relatively highelectrical resistance between central piezoresistive layer 37 and rowand column conductive threads 31, 32. Moreover, with centralpiezoresistive layer 37 uncompressed, its thickness and hence resistanceare at a maximum value.

FIGS. 6B and 6C illustrate the effects of increasing external normalforces or pressures exerted on sensor array 30. As shown in FIGS. 6B and6C, sensor array 30 is placed with its lower surface 46 supported on asurface S and a force N is exerted perpendicularly downwards on uppersurface 47 of the array, resulting in a reaction force U being exertedupwardly by supporting surface S on lower surface 46 of the array. Sincecentral piezoresistive layer 37 is resiliently deformable, thecompressive force on it decreases the thickness T of the part of thelayer between a row conductive thread 31 and a column conductive thread32. This reduction in path length through piezoresistive layer 37between a row conductive thread 31 and a column conductive thread 32causes the electrical resistance R between the threads to decrease invalue.

For moderate values of normal force N, as shown in FIG. 6B, resilientdeformation of central piezoresistive layer 37 is relatively small,resulting in a relative small reduction in electrical resistance Rbetween the threads. Larger forces N exerted on sensor array 30 cause alarger deformation of the central piezoresistive layer, as shown in FIG.6C, resulting in a larger percentage reduction in resistance R. FIG. 7illustrates in a general way the reduction in electrical resistancemeasurable between a row conductive thread 31 and a column conductivethread 32, as a function of normal force or pressure exerted on array 30at these points.

FIGS. 3 and 4 illustrate another embodiment 50 of a piezoresistivethread pressure sensor array in which the central piezoresistive layershown in FIGS. 1 and 2 and described above is replaced by apiezoresistive coating on either, or both, row conductive threads 51 andcolumn conductive threads 52.

Sensor array 50 is facially similar to sensor array 20 disclosed andshown in FIGS. 1 and 2 of U.S. Pat. No. 6,543,299, but differs from thatsensor array in important ways. Thus, row and column piezoresistivethreads 51, 52 of sensor array 50 are made of elastically stretchablepolymer cores 51C, 52C which have been treated by silver plating thecores to form on the threads electrically conductive coatings 51P, 52P,respectively. The coatings on either or both cores 51C, 52C are cladwith a layer 51R, 52R, respectively, of a material which has apiezoresistive characteristic. The piezoresistive material used to formcladding layers 51R, 52R on plated surfaces 51P, 52P of cores 51C, 52C,of piezoresistive conductive threads 51, 52 may have a compositionsimilar to that described above for making piezoresistive sheet layer37.

A method for making piezoresistive sensor threads by cladding conductivethreads with a layer of a piezoresistive material includes preparing aslurry of piezoresistive material having a composition described inexamples 1, 2 and 3 above. A highly conductive polymer thread, such assilver plated nylon thread 117/17 2 ply, Cat#124 available from LESS EMFInc., 804 Madison Avenue, Albany, N.Y. 12208, is then immersed in acontainer holding the slurry, for a period of about 10 seconds. The endof a thread which has been immersed is withdrawn from the container, andwhile it is still wet, drawn through a circular aperture through ascraper plate.

In an example embodiment, a conductive thread having a core diameter of0.25 mm and wet-coated diameter in the range of about 0.4 mm to 0.5 mmwas drawn through a #360 scraper having a diameter of 0.45 mm, thusresulting in a wet scraped diameter of about 0.45 mm. The scraped threadwas then fed through a stream of air heated to a temperature of 70degrees C. at a linear travel speed of 100 mm/minute for a period of 5minutes, to thus form a solidified coating having a diameter of about0.4 mm.

As shown in FIGS. 3 and 4, piezoresistive row and column threads 51, 52are fastened to upper and lower substrate sheets 63, 65, by suitablemeans such as adhesive blobs 74. Substrate sheets 63, 64 are made of athin, flexible material such as 0.003 inch thick elastomericpolyurethane or polyvinyl chloride (PVC) that has a relatively highdegree of elasticity.

FIGS. 3 and 8A-8C illustrate how the arrangement of row and columnpiezoresistive threads 51, 52 of sensor array 50 form individual forcesensing elements 69. In response to progressively larger compressivenormal forces, piezoresistive cladding layers 51R, 52R on row and columnconductive core threads 51C, 52C are progressively compressed into ovalcross-sectional shapes of smaller diameter. Thus, as shown in FIGS.8A-8C, the electrical resistance of each sensor element 70 decreases ininverse proportion to applied pressure, as shown in FIG. 7.

FIG. 5 illustrates a modification 70 of the sensor arrays shown in FIGS.1 and 3 and described above. Modified sensor array 70 may alternativelyemploy the three-layer construction of sensor array 30 shown in FIG. 1,or the two-layer construction of sensor array 50 shown in FIG. 3. Themodification consists of fabricating sensor array 70 with electricallyinsulating material between adjacent rows and/or columns of conductivethreads. Thus, for example, the modification 70 of two-layer sensor 50shown in FIG. 3 includes elongated insulating threads 71, made forexample of 0.012 inch diameter polyester disposed between each pair ofadjacent row conductive threads 51 and each pair of adjacent columnconductive threads 52.

The insulating threads 71 are secured in place by any suitable means,such as adhesively bonding the threads to substrate sheets 63, 65 (seeFIGS. 2 and 4). This constructing enables sensor array 70 to besubstantially wrinkled or otherwise deformed to conform to anirregularly shaped surface, without the possibility of pairs adjacentrow or column conductive threads 51 or 52 contacting one another to thuscause an electrical short circuit which would result in erroneous sensorelement resistance measurements and force determinations. Optionally,insulation between adjacent pairs of row and column conductive threadscould be applied by lightly spraying an aerosol insulation acrylic paintto hold the conductive threads in place.

FIGS. 9-12 illustrate a three-layer embodiment 80 of a piezoresistivethread force sensor array. Sensor array 80 is similar to the basicembodiment 30 of sensor array shown in FIGS. 1-2 and described above.However, sensor array 80 uses upper and lower substrate sheets 83, 85which are made of woven fabric rather than polymer films. Thisconstruction, in conjunction with the use of stretchy conductive row andcolumn threads 81, 82 made of plated nylon or Lycra cores, results in asensor array that is even more flexible, elastically stretchable anddrapable than sensor array 30.

As may be seen best by referring to FIG. 10, sensor array 80 includes aplurality of parallel, laterally spaced apart conductive row threads 81which are fastened to the lower surface 84 of upper fabric substratesheet 83. The row conductive threads 81 are fastened to lower surface 84of upper substrate sheet 83 by any suitable means. In one embodiment, asshown in FIG. 10, each row conductive thread 81 is fastened to asubstrate sheet by sewing the thread to fabric substrate sheet 83 by asmaller diameter, non-conductive thread 90 arranged in an elongatedzig-zag stitching pattern. In an example embodiment, threads 90consisted of 0.005-0.010 inch diameter, 100% polyester woven thread. Forgreater strength required for sensor arrays used to measure largerforces, threads 90 may optionally be monofilaments.

In an example embodiment of a sensor array 80, upper and lower substratesheets 83, 85 were made from a light-weight, elastically stretchablefabric, both of the two following fabrics were tested and found suitablefor substrate sheets 83, 85. (1) Milliken “Mil/glass” brand, Style#247579, composed of 69% nylon, 31% spandex, and having a weight of 1.8oz./sq. yd. (2) Milliken “Interlude” brand, product #247211, composed of82% nylon, 18% Lycra, and having a weight of 3.2-3.4 oz. Per sq. yd.Both of the foregoing fabrics are available from Milliken & Company, 23Fiddler's Way, Lafayette, N.J. 07848.

As shown in FIG. 11, lower column conductive threads 82 are fastened tothe upper surface 86 of lower fabric substrate sheet 85 bynon-conductive threads 91 of the same type as non-conductive threads 90and in the same zig-zag stitching manner.

As shown in FIGS. 9 and 12, three-layer fabric substrate sensor array 80includes a central piezoresistive sheet 87, which may have a compositionand construction similar to that of central piezoresistive sheet 37 ofsensor array 30 described above.

As may be seen best by referring to FIG. 13B, upper, row piezoresistivethreads 101 are attached to lower surface 114 of upper fabric substratesheet 113 by insulating sewn threads 90 arranged in zig-zag stitches.Similarly, lower, column piezoresistive threads 102 are attached to theupper surface 116 of lower substrate sheet 115 by sewn threads 91arranged in zig-zag stitches.

FIGS. 13A and 13B illustrate another two-layer embodiment 100 of apiezoresistive thread force sensor array. Sensor array 100 is similar tosensor array 80. However, in sensor array 100, conductive row and columnthreads 81, 82 are replaced by piezoresistive threads 101, 102 whichhave the same characteristics as piezoresistive threads 51, 52 of thetwo-layer polymer film substrate sensor array 50 shown in FIGS. 3 and 4and described above. This construction eliminates the requirement forthe central piezoresistive sheet 87 of three-layer fabric sensor array80 described above.

FIGS. 14-16 illustrate a fifth, single layer embodiment 120 of a forcesensor array in which row and column piezoresistive threads are attachedto a single side of a single insulating fabric substrate sheet 127.

As shown in FIGS. 14-16, single layer fabric force sensor array 120 hasa single substrate sheet 127 which is made from a light-weight,elastically stretchable fabric. Both of the two following fabric werelisted and found suitable for making substrate sheet 127. (1) Milliken“Millglass” brand, Style #247579, composed of 69% nylon, 31% spandex,and having a weight of 1.8 oz./sq. yd., and (2) Milliken “Interlude”brand, product #247211, composed of 82% nylon, 18% Lycra, and having aweight of 3.2-3.4 oz. Per sq. yd. Both of the foregoing fabrics areavailable from Milliken & Company.

A plurality of parallel, laterally spaced apart column piezoresistivethreads 122 are fastened to the upper surface 130 of the substratesheet. The column piezoresistive threads are made from silver-platednylon thread, Catalog #A-264 obtained from LESS EMF, or fromsilver-plated stretchy nylon yarn, both of which are described in detailabove in conjunction with the description of sensor array 30.

In one embodiment of single fabric substrate sheet sensor array 120,each column piezoresistive thread 122 is fastened to substrate sheet 127by a smaller diameter, non-conductive thread 91 arranged in an elongatedzig-zag stitching pattern. In an example embodiment, threads 91consisted of 0.005-0.010 diameter, 100% polyester.

As shown in FIGS. 14, 15 and 16, sensor array 120 includes a pluralityof parallel, laterally spaced apart piezoresistive row threads 121 whichare also fastened to the upper surface 130 of substrate sheet 127. Asshown in FIG. 16, m row piezoresistive threads 121 are fastened tosubstrate sheet 127 by non-conductive threads 90 of the same type asthreads 91 and in the same zig-zag stitching manner.

As shown in FIG. 16, opposed inner facing outer surface 128, 129 of rowand column piezoresistive threads 121, 122 tangentially contact eachother. Thus, as shown in FIGS. 14-16, each crossing of a rowpiezoresistive thread 121 with a column piezoresistive thread 122 formsa piezoresistive sensor element 138 which consists of a small portion ofpiezoresistive coatings of a row and column piezoresistive threadtangentially contacting one another.

FIG. 17 illustrates a modification of the force sensor arrays usingfabric substrate sheets shown in FIG. 9, 13 or 14 and described above.As shown in FIG. 17, a lower fabric substrate sheet 145 of modifiedforce sensor array 140 has attached thereto lower, column conductivepiezoresistive threads 142 which are sinuously curved with respect toparallel straight base lines between opposite ends of each thread,rather than lying directly on the base lines, as are the columnconductive threads 82 of sensor array 80 shown in FIG. 11. With thisarrangement, lower fabric substrate sheet 145 is even more readilyelastically stretchable in directions parallel to the column thread baselines because longitudinally spaced apart points on the fabric substratesheet are not constrained to be at maximum lengths by the lesselastically stretchable conductive threads. Thus, the stretchability ofthe column substrate sheet 145 is limited only by its intrinsicstretchability since the arrangement of column conductive threads 142allows them to conform readily to size of the substrate sheet bychanging spacing between peaks and valleys of the sinuously curvedconductive threads, i.e., altering the spatial wavelengths of thesinuous curves formed by threads.

Optionally, upper row piezoresistive threads 141 may also be sinuouslyarranged in the same manner as lower column piezoresistive threads shownin FIG. 17, to thus enhance elastic compliance, or stretchability, ofsensor array 140 is in directions parallel to the row conductive threadsas well as in directions parallel to the column piezoresistive threads.Also, either or both row and column conductive threads of three-layersensor arrays such as those of the type shown in FIG. 1 may be sinuouslyarranged to provide enhanced uniaxial or biaxial stretchability.

FIGS. 18-21 illustrate another modification 180 of the single fabricsubstrate sheet sensor array 120 of FIG. 14. Sensor array 180 has upper,row conductive threads 181 and lower, column conductive threads 182which are both sinuously arranged on opposite sides of a fabricpiezoresistive central substrate sheet 187. This construction givesarray 180 greater elasticity in directions parallel to the columnconductive threads 182 as well as in directions parallel to rowconductive threads 181.

FIG. 21A illustrates another modification 200, which row and columnpiezoresistive threads 201, 202 are both sinuously arranged and attachedto the upper surface 211 of an insulating substrate sheet 210, in themanner shown in FIG. 16.

FIG. 22A illustrates the number of conductive leads required to measurethe resistance of individual elements of a linear array of sensorelements, to thus determine numerical values of force or pressureexerted on each sensor element. As shown in FIG. 22A a single commonlead-out conductor C is connected to a linear array of intersectinglead-out conductors Li through Ln to form a plurality of sensor elementsSI through Sn, by piezoresistive material at each intersection point.Thus, for a total of n sensors S, there are required a total R equal ton+1 lead-out conductors to measure the individual resistance of eachsensor element SI through Sn and hence determine the forces F1 throughFn exerted on each individual sensor element.

FIG. 22B shows a plurality of sensor elements Sn+1, Sn+2, Sn+3 which arenot necessarily arranged in a linear array, being located, for example,on individual finger tips. As shown in FIG. 22B, n+1 lead-out conductorsare also required for this configuration.

FIG. 7 illustrates the electrical resistance of a one-inch squarepiezoresistive force sensor element 48 using a piezoresistive sheet 37having the formulation listed for an example sensor array 30 shown inFIGS. 1 and 2, and fabricated as described above, as a function ofnormal force or pressure exerted on the upper surface 47 of uppersubstrate sheet 33 of sensor array 30. As shown in FIG. 7, theresistance varies inversely as a function of normal force.

As shown in FIG. 1, row conductive threads 31-1 through 31-m, invertical alignment with column conductive threads 32-1 through 32-n formwith piezoresistive layer sheet 37 between the column and row conductivethreads a mXn rectangular matrix array of m×n force elements 48.

If upper and lower electrical connections to each sensor element 48 wereelectrically isolated from connections to each other sensor element, aseparate pair of lead-out conductors for each of the sensors, would berequired, i.e., a total of 2Qlead-out conductors for Q sensor elementsor, if a single common electrode lead-out were employed as shown in FIG.22, a total of Q+1 lead-outs would be required.

As shown in FIG. 1, sensor array 30 is arranged into a matrix of m rowsand n columns, thus requiring only R=m×n lead-out conductors. However,as shown in FIG. 23, if matrix addressing of sensor array 30 is used tomeasure the resistance of individual sensors 48 to thereby determinenormal forces exerted on the sensors, there is a substantial cross-talkbetween the resistance on an addressed sensor 48 and non-selectedsensors because of parallel current paths to non-addressed sensors. Toovercome this cross-talk problem, the present inventor has developed amethod for modifying sensors 48 to give them a diode-likecharacteristic. As may be confirmed by referring to FIG. 24, thecross-talk between sensor elements 40 which have a non-bilateral,polarity-sensitive transfer function, mitigates the cross-talk problempresent in the matrix of symmetrically conductive sensors 48 shown inFIG. 23.

Sensor elements 48 are modified to have a diode-like characteristic bymodifying the preparation of piezoresistive layer sheet 37, as follows:First, a piezoresistive layer sheet 37 is prepared by the processdescribed above. Then, either the upper surface 40 or the lower surface41 of the piezoresistive coating 37A of piezoresistive sheet 37 ismodified to form thereon a P-N, semiconductor-type junction.

Modification of piezoresistive coating 37A to form a P-N junction isperformed by first preparing a slurry which has the composition of oneof the three example mixtures described above, but modified by theaddition of 5 ml each of copper oxide (CuO) in the form of a fine powderof 50-micron size particles, and 5 ml of cuprous oxide (Cu₂O) in theform of a fine powder of 50-micron size particles and thoroughlystir-mixing the foregoing ingredients. The resultant solution is thenreduced using about 30 mg of solution of sodium borohydride, also knownas sodium tetrahydroborate (NaBH₄) or ammonium phosphate, to form asolution having a pH of about 5.5. The solution is then coated onto theupper surface 40 or lower surface 41 of piezoresistive coating 37B onpiezoresistive sheet 37. This coating process is performed using aroller coating process which results in about 0.5 ml of solution persquare centimeters being applied. The surface coating is then allowed toair-dry at room temperature and a relative humidity of less than 20%,for 4 hours. After the coated surface has dried, it functions as aP-type semiconductor, while the uncoated side of coating 37B functionsas an N-type semiconductor of P-N junction diode.

FIG. 29 illustrates a sensor element 48 which has been prepared asdescribed above to give the sensor a diode-like characteristic, and acircuit for obtaining the I-V (current versus voltage) transfer functionof the sensor. FIG. 30 shows a typical I-V curve for sensor elements 48of FIG. 29.

As stated above, the advantage of modifying sensor elements 48 of sensorarray 30 by adding a semi-conductive layer that acts like a diode isthat it reduces cross talk between sensors. As is shown in FIG. 23, thiscross-talk occurs because of the so-called “completing the square”phenomenon, in which three connections are made in a square matrix arrayof three non-addressed resistors that form the three corners of asquare. Thus, any two connections in a vertical column and a third onein the same row function as either connection in an X-Y array ofconductors. The resistor at the fourth corner of the square shows up asa phantom in parallel with an addressed resistor because the current cantravel backwards through that resistor, and forward through the otherresistors. Care and additional expense must be taken in the electronicsto eliminate the contribution of this phantom. For example, if, as isshown in FIG. 23, a potential V is applied between row and columnconductors X₁Y₁, to thereby determine the resistance of piezoresistivesensor resistance R₁₁, reverse current flow through “phantom” resistorR₂₂ would cause the sum of resistances R₁₂+R₂₂+R₂₂ to shunt R₁₁,resulting in the parallel current flow paths indicated by arrows in FIG.23, which in turn would result in the following incorrect value ofresistance:

R_(x1)y₁=R₁₁//(R₁₂+[R₂₂]+R₂₁),R_(x1)Y₁=R₁₁(R₁₂+[R₂₂]+R₂₁)/(R₁₁+R₁₂+[R₂₂]+R₂₁), where brackets around aresistance value indicate current flow in a counterclockwise directionthrough that resistor, rather than clockwise, i.e., diagonally downwardstowards the left. Thus, for example, if each of the four resistanceslisted above had a value of 10 ohms, the measured value of R₁₁ would be:

R₁₁=10(10+10+10)/(10+10+10+10)=300/40=7.5 ohms, i.e., 25% below theactual value, 10 ohms, of R₁₁. If the resistance values of R₁₂, R₂₂ andR₂₁ of the three non-addressed piezoresistive sensor element 48 wereeach lower, e.g., 1 ohm, because of greater forces concentrated on thosesensor elements 48, the measured value of R₁₁ would be:

R₁₁=10(1+1+1)/(10+1+1+1)=30/13=2.31 ohms, i.e., a value of about 77percent below the actual value of R₁₁.

On the other hand, by placing a diode in series with each piezoresistivesensor element 48, as shown in FIG. 24, the electrical resistance of anelement measured in a reverse, counterclockwise direction a test currentflow through the sensor element, e.g., R₂₂, would be for practicalpurposes arbitrarily large, or infinity compared to the clockwiseforward paths of current through the other resistances shown in FIGS. 23and 24. In this case, the measured resistance value for a 2×2 matrix offour resistances each having a value of 10 ohms would be:

R_(X1)Y₁=10(1+∞+1)/(10+1+∞+1)=10 ohms, the correct value. Thus,modifying each sensor element 48 to include a p-n junction thereby givethe sensor element a diode-like characteristic electrically isolates,i.e., prevents backward current flow, through each sensor element 48.This enables the correct value of electrical resistance of each sensorelement 48 and hence forces exerted thereon to be measured accuratelyR_(X1)y₁ using row and column matrix addressing rather than requiring aseparate pair of conductors for each sensor element.

FIG. 25 illustrates a force measuring apparatus 150. The apparatus 150may use any of the types of sensor arrays described above, but in aparticular example shown in FIG. 25 uses a sensor array 70 of the typeshown in FIG. 5.

As shown in FIG. 25, force measuring apparatus 150 used four sensorarrays 70-1,70-2, 70-3 and 70-4, each having a matrix of 16 rowconductive threads by 16 column conductive threads. The four arrays arearranged in a square matrix, to thus form a composite sensor array 70-Cconsisting of 32 rows×32 columns of conductive threads having formed attheir intersection 32×32=1,024 sensor elements 88. As shown in FIG. 25,each of the 32 row conductive thread lead-out wires and each of the 32column conductive thread lead-outs is connected to a separateelectrically conductive connector pin of a plurality of connector pins154-1 through 154-64 of a pair of electrical interface connectors 153-1,153-2.

FIG. 26 illustrates a force measurement system 160 which utilizes theforce sensor apparatus 150 described above.

As shown in FIG. 26, force measurement system 160 includes a computer161 which is bidirectionally coupled to force sensor array 70 of forcesensor apparatus 160 through a force sensor interface module 162. Thesensor interface module 162 includes a Digital-to analog Converter (DAC)163 for generating in response to control signals from computer 161 testvoltages or currents which are directed to matrix-addressed individualforce sensors 88.

As shown in FIG. 26, individual force sensor elements 88 are addressedby connecting one terminal of a current or voltage source controlled byDAC 163 to a selected one of X-row conductors 51-1-51-m by an Xmultiplexer 164, and connecting the other terminal of the source to aselected one of Y-column conductors 52-1-52-m by a Y multiplexer 165.Sensor interface module 162 also included an Analog-to-Digital Converter(ADC) 166 which measures the voltage drop or current through a sensorelement 88 resulting from application of a test current or voltage, andinputs the measured value to computer 161. Using predetermined scalefactors, computer 161 calculates the instantaneous value of electricalresistance of a selected addressed sensor element 88, and from thatresistance value, a corresponding normal force instantaneously exertedon the addressed sensor.

In response to control signals cyclically issued by computer 161, Xmultiplexer 164 and Y multiplexer 165 are used to cyclically measure theresistance of each force sensor element 88, at a relatively rapid rateof, for example, 3,000 samples per second, enabling computer 161 tocalculate the force exerted on each force sensor element 88 at thatsampling rate.

Measurement system 160 includes an operator interface block 167 whichenables values of force or pressures measured by sensor elements 88 tobe displayed as numerical values and/or a graph or pressure/force map onthe display screen of a computer monitor 168, or outputted to aperipheral device such as a printer, or a network such as the internet,through an I/O block 169.

FIGS. 27A and 27B illustrate a sock 170 which includes one of the novelsensor arrays employing conductive threads which were described above,such as the single layer, fabric substrate piezoresistive thread sensorarray shown in FIG. 14-16 or 17-20.

As shown in FIG. 17, sock 170 which includes a single layer fabric forcesensor array 180 that is a modification of the planar force sensor array120 shown in FIGS. 14-16 and described above. The modification of forcesensor array 120 to form force sensor array 180 may be best visualizedby considering that the left and right side edges of the array 120 arebrought upwards from the plane of the page to meet and form a hollowcylindrical tube.

Row conductor threads protruding 121 from the aligned edges of the arrayare then electrically conductively fastened to a first, row conductorribbon cable 181. Column conductive threads protruding from one edge ofthe rolled-up array are electrically conductively fastened to a second,column conductor ribbon cable 182. Outer ends 183, 184 who protrude froman edge of array 120 are electrically connected to a resistancemeasuring circuit as shown in FIG. 26 and described above.

FIGS. 31-34 illustrate modifications of fabric substrate force sensorarrays using conductive threads, in which the conductive threads arefixed to a fabric substrate sheet without the use of sewn stitching byadhesive applied directly to a conductive thread. Thus, a first,three-layer fabric sensor array 190 includes a plurality of parallel,spaced apart row conductive elastic threads 191 which are adhesivelybonded to the lower surface 194 of an upper stretchable fabric substratesheet 193 made of 3 mil thick polyester or either of the two Millikenfabrics described above. Sensor array 190 also includes a plurality ofparallel spaced apart column conductive elastic threads 192 which areadhesively bonded to an upper surface 196 of a lower stretchable fabricsubstrate sheet 195. A thin sheet of stretchable fabric prepared to giveit a piezoresistive property in the manner described above comprises acentral piezoresistive layer 197 which is positioned between row andcolumn conductive threads 191, 192. The foregoing three layers are thenstacked on top of one another and dots of glue injected through the meshopenings of the fabric substrate of all three layers to adhere themtogether and thus form a completed sensor array 190.

Sensor array 200, shown in FIG. 33, utilizes a single substrate sheet207. Conductive row and column threads 191, 192, separated by insulatingthreads 210, 211, are adhered to upper surface 212 and lower surface 213of sheet 207 by double-stick tape strips 213, 214.

FIGS. 35-43 illustrate various aspects of a method and apparatus forminimizing body force concentrations on a human body using an adaptivecushion. The example embodiment depicted in FIGS. 35 and 37 includes anadaptive cushion which is of an appropriate size and shape for use on astandard single or hospital bed. However, as will be clear from theensuing description of that example embodiment, the size and shape ofthe adaptive cushion can be varied to suit different applications, suchas for use on a fixed chair or wheel chair.

Referring first to FIGS. 35 and 36A, an adaptive cushion apparatus 420for minimum body force concentrations on a body of a person lying on abed may be seen to include a longitudinally elongated, rectangularcushion overlay 421. Cushion 421 has an appropriate size and shape tofit conformally on top of a standard size hospital bed. Thus, an exampleembodiment of cushion 421 had a laterally elongated, rectangular shapewith a length of about 6 feet, a width of about 3 feet, and a thicknessof about 4 inches.

The six panels of each air bladder cell 423 are sealingly joined atedges thereof to form a hermetically sealed body which has a hollowinterior space 422A.

As shown in FIG. 36A, mattress overlay cushion 421 is constructed as arectangular, two-column by six-row array of 12 individual inflatable airbladder cells 422. Each air bladder cell 422 has a laterally elongated,rectangular shape, having a length of about 18 inches, a depth of about17 inches, and a thickness of about 4 inches. As shown in FIGS. 35 and36, bladders 422 are arranged in left and right columns, each having 6longitudinally spaced apart, laterally disposed, laterally elongatedbladders. As shown in FIGS. 36 and 38, each air bladder cell has a flatbase panel 423, left and right end panels 424, 425, head and toe orfront and rear panels 426, 427, and an upper panel 428. The bladders 422are made of a thin sheet of a flexible, elastomeric material such asneoprene rubber or polyurethane, having a thickness of about 0.014 inch.The six panels of each air bladder cell 422 are sealingly joined atedges thereof to form a hermetically sealed body which has a hollowinterior space 422A. Optionally, each air bladder cell 422 may befabricated from a tubular preform in which each end panel is sealinglyjoined to opposite transverse ends of the tubular preform. In eitherembodiment, adjacent panels of an individual air bladder cell aresealingly joined by a suitable method such as ultrasonic bonding,RF-welding or adhesive bonding.

The number, size, shape, relative positioning and spacing of air bladdercells 422 of mattress cushion overlay 421 are not believed to becritical. However, it is believed preferable to arrange mattress overlay421 into symmetrically-shaped left and right columns each having atleast five and preferably six longitudinal zones corresponding to majorcurvature of a longitudinally disposed medial section of a typical humanbody. Thus, as shown in FIGS. 35, 36A and 37, mattress overlay cushion421 has a left-hand column of six air bladder cells 422L1-422L6, and aright-hand column of six cells 421R1-421R6.

As shown in FIGS. 38 and 40, the bladders are stacked closely togetherin both front and rear and side by side directions, with minimumlongitudinal and lateral spacings 429, 430, respectively, that arevanishingly small so that adjacent bladder cells physically contact eachother.

As indicated in FIGS. 35 and 36, each bladder cell 422 is provided witha tubular air inlet port 431 which protrudes through a side wall, e.g.,a left or right side wall 424 or 425, and communicates with a hollowinterior space 422A within the bladder. Air admitted into or exhaustedfrom hollow interior space 422A through port 431 of an air bladder cell422 enables the cell to be inflated or deflated to a selected pressure.

Although the shape of each air bladder cell 422 of cushion 421 shown inFIGS. 35 and 36 is that of a rectangular block, or parallelepiped, theair bladder cells may optionally have different shapes, such as convexhemispheres protruding upwards from the base of the cushion. Also, thearray of air bladder cells 422 of cushion 421 may be parts of a unitarystructure with a common base panel 423 which has individualrectangular-block shaped, hemispherical or hollow inflatable bodies ofother shapes protruding upwardly from the common unitary base panel.

Whether individual air bladder cells 422 are separate bodies or upperinflatable shell-like portions protruding upwardly from a common base,air inlet/exhaust port tubes 431 of each air bladder cell 422, orselected air bladder cells 422, may be located in the base panel 423 ofthe cell and protrude downwardly from the cell, rather than beinglocated in a side wall and protruding laterally outwards, as shown inFIGS. 35 and 36A.

As shown in FIGS. 35, 36 and 39, body force minimization apparatus 420includes a force sensor array 432 which has a matrix of individual forcesensors 433, with at least one sensor positioned on the upper surface428 of each air bladder cell 422. As will be explained in detail below,each force sensor 433 comprises a force sensitive transducer which hasan electrical resistance that varies inversely with the magnitude of anormal, i.e., perpendicular force exerted on the sensor by an objectsuch as the body of a person supported by overlay cushion 421. In oneembodiment, force sensor array 432 is maintained in position on theupper surfaces of air bladder cells 422 by a water-proof, form-fittingcontour fabric sheet 421A which fits tightly and removably over cushion421, as shown in FIG. 37.

Referring to FIG. 35, it may be seen that body force minimizationapparatus 420 includes an electronic control module 435. As will beexplained in detail below, electronic control module 435 includes sensorinterface circuitry 436 for electrical interconnection to sensors 433.Electronic control module 435 also includes a computer 437 which isinterconnected with sensor interface circuitry 436. Computer 437 isprogrammed to receive input signals from sensor interface circuitry 436,measure the resistance of individual sensors 433 and calculate therefromthe magnitude of forces exerted on each sensor, make calculations basedon the force measurements, and issue command signals to control thepressure in individual air bladder cells 422 which are calculated usingan algorithm to minimize force concentrations on the cells.

In one embodiment of apparatus 420, measurement of the resistance ofeach sensor 433 is facilitated by arranging the sensors into a matrixarray of rows and columns. With this arrangement, individual resistancesof a 6×2 array 432 of sensors 433 may be measured using 6 row interfaceconductors and 2 column interface conductors 450, 451, as shown in FIG.35.

To avoid cross talk between measurements of individual sensors 433, theaforementioned row-column addressing arrangement requires that eachsensor have a non-bilateral, asymmetric current versus voltagecharacteristics, e.g., a diode-like impedance characteristic. As will bedescribed in detail below, the present invention includes a novel sensorhaving the required diode-like characteristic. Alternatively, usingforce sensors 433 which do not have a diode-like characteristic, theforce sensor array 432 can be partitioned into 12 separate rectangularsensors 433 each electrically isolated from one another, with a separatepair of interface conductors connected to upper and lower electrodes ofeach sensor.

As shown in FIG. 35, body force minimization apparatus 420 includes anair pump or compressor 440 for providing pressurized air to the inputport 442 of a selector valve manifold 441. Selector valve manifold 441has 12 outlet ports 443A, each connected through a valve 443 to aseparate air bladder cell inlet port 431. As will be explained in detailbelow, the compressor 440, selector valve manifold 441 and valves 443are operably interconnected to computer 437 and an air pressuremeasurement transducer 444. Pressure transducer 444 outputs anelectrical signal proportional to pressure, which is input to computer437. This arrangement enables the inflation pressure of each air bladdercell 422 to be individually measured and varied under control of thecomputer 437.

FIGS. 36A, 38 and 39 illustrate details of the construction of forcesensor array 432. As shown in those figures, sensor array 432 includesan upper cover sheet 445 made of a thin flexible, elasticallystretchable material. In an example embodiment of sensor array 432fabricated by the present inventor, cover sheet 445 was made of “two-waystretch” Lycra-like material which had a thickness of about 0.010 inchand a thread count of about 88 threads per inch. That material had thetrade name Millglass Platinum, Style No. (24)7579, obtained from theMilliken & Company, P.O. Box 1926, Spartanburg, S.C. 29304.

Referring to FIG. 39, sensor array 432 includes an upper, columnconductor sheet 446 which is fixed to the lower surface of upperflexible cover sheet 445, by flexible adhesive strips made of 3Mtransfer tape 950, or a flexible adhesive such as Lepage's latex contactadhesive. Column conductor sheet 446 is made of a woven fabric matrixsheet composed of 92% nylon and 8% Dorlastan fibers, which give thesheet a flexible, two-way stretch elasticity. The fabric matrix sheet ofconductor sheet 446 is electroless plated with a base coating of copper,followed by an outer coating of nickel. The metallic coatings completelyimpregnate the surfaces of fibers adjacent to interstices of the meshfabric, as well as the upper and lower surfaces 447, 448 of theconductor sheet 446, thus forming electrically conductive paths betweenthe upper and lower surfaces 447 and 448. The present inventor has foundthat a suitable conductive fabric for conductor sheet is a Woven Silverbrand, Catalog #A251 available from Lessemb Company, 809 Madison Avenue,Albany, N.Y. 12208, USA.

In an example embodiment of sensor array 432, upper conductive sheet 446was fabricated from the Woven Silver, Catalog #A151 material describedabove. The surface resistivity of upper and lower surfaces 447, 448 ofthat material was about 1 ohm per square or less, and the inter-layerresistance between upper and lower surfaces 447, 448 was about 50 ohmsper square.

In one embodiment of sensor array 432, individual conductive pads, orrows or columns of conductors, are formed by etching metal-free channelsvertically through conductor sheet 446, from the top of upper conductivesurface 447, all the way to the bottom of lower conductive surface 448.Thus, as shown in FIG. 39, narrow longitudinally disposed straightchannels 449 are etched through upper column conductor sheet 446. Thisconstruction results in the formation of two adjacent, relatively wide,longitudinally elongated left and right planar column electrodes 450,451. The adjacent left and right column electrodes are separated by arelatively thin channel 449, thus electrically isolating the adjacentcolumn electrodes from each other.

Insulating channels 449 are etched through upper conductor sheet 446 toform column electrodes 450 and 451 by the following novel process.

First, to prevent capillary wicking and resultant wetting of asubsequently applied etchant solution to fabric conductor sheet 446, thesheet is pre-processed by treating it with a hydrophobic substance suchas PTFE. The treatment can be made by spraying the conductor fabricsheet 446 with an aerosol containing a hydrophobic material such asPTFE. A suitable aerosol spray is marketed under the trade name ScotchGuard by the 3M Company, St. Paul, Minn. Areas of fabric conductor sheet446 which are to have insulating channels 449 formed therein are maskedfrom the hydrophobic treatment by adhering strips of masking tape whichhave the shape of the channels to the sheet before applying thehydrophobic material to the sheet.

Following the pre-processing of conductor sheet 446 to make ithydrophobic, sheets of masking tape are adhered tightly to both upperand lower surfaces 447, 448 of the conductor sheet, using a roller orpress to insure that there are no voids between the masking tape andsurfaces, which could allow etchant solution to contact the conductivesurfaces. Next, strips of masking tape having the shape of insulatingchannels 449 are removed from the conductor sheet. Optionally, thestrips of masking tape to be removed are preformed by die-cuttingpartially through larger sheets of masking tape.

After strips of masking tape corresponding to channels 449 have beenstripped from conductor sheet 446, the conductive metal coatings of thefabric sheet aligned with the channels is chemically etched away. Onemethod of performing the chemical etching uses a concentrated solutionof 10 mg ammonium phosphate in 30 ml of water. The ammonium phosphatesolution is mixed with methyl cellulose solid powder, at a concentrationof 10 percent methyl cellulose powder until a gel consistency isobtained. The etchant gel thus formed is then rollered onto the areas ofupper and lower surfaces 447, 448 of conductor sheet 446, over channels449. The etchant gel is allowed to reside on channels 449 forapproximately 1 hour, at room temperature, during which time the nickeland copper plating of the fabric matrix of conductor sheet 446, invertical alignment with channels 449, is completely removed, thus makingthe channels electrically insulating. This process separates theconductor sheet into left and right column electrodes 450, 451,respectively.

The etching process which forms insulating channel 449 is completed byrinsing the etchant gel from upper and lower surfaces 447, 448 ofconductor sheet 446, followed by removal of the masking tape from theupper and lower surfaces.

Referring still to FIG. 39, it may be seen that sensor array 432includes a thin piezoresistive sheet 452 which has on an upper surface453, that is in intimate contact with lower surfaces of left and rightcolumn electrodes 450, 451. Piezoresistive sheet 452 also has a lowersurface 454 which is in intimate electrical contact with the uppersurfaces of row electrodes on a lower row conductor sheet 456. Lower,row conductor sheet 456 has a construction exactly similar to that ofupper, column conductor sheet 446. Thus, lower row conductor sheet 456has upper and lower conductive surfaces 457, 458, and narrow, laterallydisposed insulating channels 459 which are positioned between and definerow electrodes 461, 462, 463, 464, 465, 466.

The function of piezoresistive sheet 452 of sensor array 432 is to forma conductive path between column and row electrodes, e.g., left-handcolumn electrode 450 and rear row electrode 461, the resistance of whichpath varies in a predetermined fashion as a function of normal forceexerted on the sensor array.

In example embodiments of sensor array 432, piezoresistive sheet 452 wasfabricated by coating a stretchy, thin Lycra-like fabric sheet with apiezoresistive material. A suitable fabric sheet, which forms a matrixfor supporting the piezoresistive material, was a fabric known by thetrade name Platinum, Milliken, Style #247579, obtained from themanufacturer, Milliken & Company, Spartanburg, S.C., USA. That fabrichad a fiber content of 69 percent nylon and 31 percent Spandex, a threadcount of about 88 threads per inch, and a thickness of 0.010 inch. Thepiezoresistive material used to coat the fabric matrix is made asfollows:

A solution of graphite, carbon powder, nickel powder and acrylic binderare mixed in proportions as required to obtain the desired resistanceand piezoresistive properties. Silver coated nickel flake is used toachieve force response in the low force range of 0 to 1 psi, graphite isused for the mid range of 1 to 5 psi and Charcoal Lamp Black is used forhigh force range of 5 to 1000 psi. Following is a description of thesubstances which are constituents of the piezoresistive material:

Silver Coated Nickel Flake:

-   -   Platelets approximately one micron thick and 5 microns in        diameter.    -   Screen Analysis (−325 Mesh) 95%.    -   Apparent Density 2.8.    -   Microtrac d50/microns 12-17.    -   Available from: Novamet Specialty Products Corporation,    -   681 Lawlins Road, Wyckoff, N.J. 07481

Graphite Powder:

-   -   Synthetic graphite, AC-4722T    -   Available from: Anachemia Science    -   4-214 DeBaets Street    -   Winnipeg, MB R2J 3W6

Charcoal Lamp Black Powder:

-   -   Anachemia Part number AC-2155    -   Available from: Anachemia Science    -   4-214 DeBaets Street    -   Winnipeg, MB R2J 3W6

Acrylic Binder:

-   -   Staticide Acrylic High Performance Floor Finish    -   P/N 4000-1 Ph 8.4 to 9.0    -   Available from: Static Specialties Co. ltd.    -   1371-4 Church Street    -   Bohemia, N.Y. 11716

Following are examples of mixtures used to make piezoresistive materialshaving different sensitivities:

Example I for forces in the range of 0 to 30 psi:

-   -   200 ml of acrylic binder    -   10 ml of nickel flake powder    -   10 ml of graphite powder    -   20 ml of carbon black

Example II for forces in the range of 0-100 psi

-   -   200 ml of acrylic binder    -   5 ml of nickel flake powder    -   5 ml of graphite powder    -   30 ml of carbon black

Example III for forces in the range of 0-1000 psi

-   -   200 ml of acrylic binder    -   1 ml of nickel flake powder    -   1 ml of graphite powder    -   40 ml of carbon black

The fabric matrix for piezoresistive sheet 452 is submerged in thepiezoresistive coating mixture. Excess material is rolled off and thesheet is hung and allowed to air dry.

FIG. 40 illustrates calculation of a minimum spacing S between adjacentair bladder cells 422, and a minimum width of non-conductive strip 449between adjacent conductors of sensor array 432.

Referring to FIG. 40, as a patient sinks into a deflating bladder 422,the upper force sensor layer 433 is drawn down and away from the bladderover which it was initially positioned. If the non-conductive strip 449is too narrow, there is a possibility that a conductor such as columnconductor 450 overlying the deflating bladder will contact adjacentconductor 451 and, thus register forces that are not representative ofthe force over the bladder in which it was originally positioned. It istherefore necessary to make the non-conductive strip 449 wide enough toprevent this from happening. If we assume a simple situation wherein anair bladder cell is deflated until the center of the cell, then theforce sensing layer is drawn down a distance equal to the diagonals (C1and C2) as shown in FIG. 40, the width S of non-conductive strip 449should be made equal to or greater than (C1+C2−the width of the bladder)to prevent forces being misread as coming from a neighboring cell.

FIG. 28 illustrates the electrical resistance of a one-inch squarepiezoresistive force sensor element 448 using a piezoresistive sheet 437having the formulation listed for an example sensor array 432 shown inFIGS. 35 and 36, and fabricated as described above, as a function ofnormal force or pressure exerted on the upper surface 447 of uppersubstrate sheet 433 of sensor array 432. As shown in FIG. 28, theresistance varies inversely as a function of normal force.

As shown in FIGS. 35 and 39, left and right column electrodes 450 and451, in vertical alignment with row electrodes 461, 462, 463, 464, 465,466, of 12 form with piezoresistive layer sheet 452 between the columnand row electrodes a 2×6 rectangular matrix array of 12 force sensors433.

Optionally, the upper and lower electrodes for each sensor 433 could besegmented into electrically isolated rectangular pads by etchingchannels 449, 459 through both upper conductive sheet 446 and lowerconductive sheet 456. This arrangement would require a separate pair oflead-out conductors for each of the 12 sensors, i.e., a total of 24leads.

As shown in FIGS. 35 and 39, sensor array is arranged into rows andcolumns, thus requiring only 8 lead-out conductors. However, as shown inFIG. 23, if matrix addressing of sensor array 432 is used to measure theresistance of individual sensors 433 to thereby determine normal forcesexerted on the sensors, there is a substantial cross-talk between theresistance on an addressed sensor 433 and nonselected sensors because ofparallel current paths to non-addressed sensors. To overcome thiscross-talk problem, the present inventor has developed a method formodifying sensors 433 to give them a diode-like characteristic. As maybe confirmed by referring to FIG. 24, the cross-talk between sensors 433which have a non-bilateral, polarity-sensitive transfer function,mitigates the cross-talk problem present in the matrix of symmetricallyconductive sensors 433 shown in FIG. 23.

Sensors 433 are modified to have a diode-like characteristic bymodifying the preparation of piezoresistive layer sheet 452, as follows:First, a piezoresistive layer sheet 452 is prepared by the processdescribed above. Then, either the upper surface 469 or the lower surface470 of the piezoresistive coating 467 of piezoresistive sheet 452 ismodified to form thereon a P-N, semiconductor-type junction.

Modification of piezoresistive coating 467 to form a P-N junction isperformed by first preparing a slurry which has the composition of oneof the three example mixtures described above, but modified by theaddition of 5 ml each of copper oxide (CuO) in the form of a fine powderof 50-micron size particles, and 5 ml of cuprous oxide (Cu₂O) in theform of a fine powder of 50-micron size particles and thoroughlystir-mixing the foregoing ingredients. The resultant solution is thenreduced using about 30 mg of solution of sodium borohydride, also knownas sodium tetrahydroborate (NaBH4) or ammonium phosphate, to form asolution having a pH of about 5.5. The solution is then coated onto theupper surface 469 or lower surface 470 of piezoresistive coating 468 onpiezoresistive sheet 452. This coating process is performed using aroller coating process which results in about 0.5 ml of solution persquare centimeters being applied. The surface coating is then allowed toair-dry at room temperature and a relative humidity of less than 20%,for 4 hours. After the coated surface has dried, it functions as aP-type semiconductor, while the uncoated side of coating 468 functionsas an N-type semiconductor of P-N junction diode.

FIG. 29 illustrates a sensor 433 which has been prepared as describedabove to give the sensor a diode-like characteristic, and a circuit forobtaining the 1-V (current versus voltage) transfer function of thesensor. FIG. 30 shows a typical 1-V curve for sensor 433 of FIG. 29.

As stated above, the advantage of modifying sensors 433 by adding asemi-conductive layer that acts like a diode is that it reduces crosstalk between sensors. As is shown in FIG. 23, this cross-talk occursbecause of the so-called “completing the square” phenomenon, in whichthree connections are made in a square matrix array of threenon-addressed resistors that form the three corners of a square. Thus,any two connections in a vertical column and a third one in the same rowfunction as either connection in an X-Y array of conductors. Theresistor at the fourth corner of the square shows up as a phantom inparallel with an addressed resistor because the current can travelbackwards through that resistor, and forward through the otherresistors. Care and additional expense must be taken in the electronicsto eliminate the contribution of this phantom. For example, if, as isshown in FIG. 23, a potential V is applied between row and columnconductors X₁Y₁, to thereby determine the resistance of piezoresistivesensor resistance R₁₁, reverse current flow through “phantom” resistorR₂₂ would cause the sum of resistances R₁₂+R₂₂+R₂₂ to shunt R₁₁,resulting in the parallel current flow paths indicated by arrows in FIG.23, which in turn would result in the following incorrect value ofresistance:

R_(x1)y₁=R₁₁//(R₁₂+[R₂₂]+R₂₁),R_(x1)Y₁=R₁₁(R₁₂+[R₂₂]+R₂₁)/(R₁₁+R₁₂+[R₂₂]+R₂₁), where brackets around aresistance value indicate current flow in a counterclockwise directionthrough that resistor, rather than clockwise, i.e., diagonally downwardstowards the left. Thus, for example, if each of the four resistanceslisted above had a value of 10 ohms, the measured value of R₁₁ would be:

R₁₁=10(10+10+10)/(10+10+10+10)=300/40=7.5 ohms, i.e., 25% below theactual value, 10 ohms, of R₁₁. If the resistance values of R₁₂, R₂₂ andR₂₁ of the three non-addressed piezoresistive sensors 433 were eachlower, e.g., 1 ohm, because of greater forces concentrated on thosesensors 433, the measured value of R₁₁ would be:

R₁₁=10(1+1+1)/(10+1+1+1)=30/13=2.31 ohms, i.e., a value of about 77percent below the actual value of R₁₁.

On the other hand, by placing a diode in series with each piezoresistivesensor element 433, as shown in FIG. 24, the electrical resistance of anelement measured in a reverse, counterclockwise direction a test currentflow through the sensor element, e.g., R₂₂, would be for practicalpurposes arbitrarily large, or infinity compared to the clockwiseforward paths of current through the other resistances shown in FIGS. 23and 24. In this case, the measured resistance value for a 2×2 matrix offour resistances each having a value of 10 ohms would be:

R_(x1)y₁=10(1+∞+1)/(10+1+∞+1)=10 ohms, the correct value.

Thus, modifying each sensor 433 element to include a p-n junctionthereby gives the sensor element a diode-like characteristic thatelectrically isolates, i.e., prevents backward current flow, througheach sensor element 433. This enables the correct value of electricalresistance R_(x1)y₁ of each sensor element 433 and hence forces exertedthereon to be measured accurately using row and column matrix addressingrather than requiring a separate pair of conductors for each sensorelement.

The above-described components of force minimization apparatus 420 areinterconnected to form a closed-loop servo control system. That systemis effective in reducing body force concentrations using an algorithmaccording to the method described herein. An understanding of thismethod and apparatus may be facilitated by referring to FIG. 41, whichis a block diagram of an electro-pneumatic controller system component420A of apparatus 420, in conjunction with the diagrammatic view of theapparatus shown in FIG. 35, and the perspective view shown in FIG. 39.

Referring to FIG. 41, it may be seen that electro-pneumatic controllerapparatus 420A includes a computer 37 which is bidirectionally coupledto force sensor array 432 through force sensor interface module 436. Thesensor interface module 436 includes a Digital-to-Analog Converter (DAC)471 for generating in response to control signals from computer 437 testvoltages or currents which are directed to matrix addressed individualforce sensors 433.

Individual force sensors 433 are addressed by connecting one terminal ofa current or voltage source controlled by DAC 471 to a selected one ofX-row conductors 1-6 by an X multiplexer 472, and connecting the otherterminal of the source to a selected one of Y-column conductors 1 or 2by a Y multiplexer 473. Sensor interface module 437 also included anAnalog-to-Digital Converter (ADC) 474 which measures the voltage drop orcurrent through a sensor 433 resulting from application of a testcurrent or voltage, and inputs the measured value to computer 437. Usingpredetermined scale factors, computer 437 calculates the instantaneousvalue of electrical resistance of a selected addressed sensor 433, andfrom that resistance value, a corresponding normal force instantaneouslyexerted on the addressed sensor.

In response to control signals cyclically issued by computer 437, Xmultiplexer 472 and Y multiplexer 473 are used to cyclically measure theresistance of each force sensor element 433, at a relatively rapid rateof, for example, 3,000 samples per second, enabling computer 437 tocalculate the force exerted on each force sensor 433 at that samplingrate.

Referring still to FIG. 41, apparatus 420 includes a pressure controlmodule 475 for dynamically controlling the air pressure in eachindividual air bladder cell 422, in response to command signals issuedby computer 437, based upon values of force measured by sensor array 432and an algorithm programmed in the computer. As shown in FIG. 41,pressure control module 475 is operably interconnected to air compressor440 and air pressure transducer 444 at output port 476 of the compressorto pressurize air in the outlet port to a value controllable by computer437.

Outlet port 476 of compressor 440 is coupled to inlet port 442 of a12-outlet port manifold 441. In response to electrical control signalsissued by computer 437 and routed through pressure control module 475,each of 12 individual air bladder cell inlet selector valves 443connected to separate outlet ports 443A of manifold 441 is individuallycontrollable.

In a first, open position of a selector valve 443, the air inlet port431 of a selected air bladder cell 422 is pressurized to a pressuremeasured by transducer 444 to a predetermined value, by turning oncompressor 440, to thereby inflate the cell to a desired pressure.Alternatively, with compressor 440 in an off-mode, a vent valve 477coupled to the input port 442 of manifold 441 may be opened to deflatean air bladder cell 422 to a lower pressure value by exhausting air tothe atmosphere.

After a selected one of the 12 selector valves 443 has been opened inresponse to a command signal from computer 437 for a time periodsufficient to inflate a selected air bladder cell 422 to a predeterminedpressure, an electrical signal output by pressure transducer 444, whichis proportional to the pressure in that cell and input to computer 437,results in the computer outputting a closure command signal to the valveand a shut-off command signal to compressor 440.

When vent valve 477 and a selected selector valve 443 have been openedin response to command signals from computer 437 to deflate a selectedair bladder cell 422 to a lower predetermined pressure, an electricalsignal from pressure transducer 444 input to computer 437 results in anelectrical closure command signal being output from the computer. Thatcommand signal closes vent valve 477 and the open selector valve 443,thereby maintaining the selected lower pressure in the selected airbladder cell. In an exactly analogous fashion, the air pressure in eachother air bladder cell 422 is sequentially adjustable by sending acommand signal to a selector valve 443 to open that valve, and operatingcompressor 440 and/or vent valve 477 to inflate or deflate the airbladder cell to a predetermined pressure.

FIG. 42 is a simplified perspective view of an embodiment of a housingfor electro-pneumatic apparatus 420A shown in FIG. 41 and describedabove. As shown in FIGS. 41 and 42, electro-pneumatic controller 420Aincludes an operator interface module 478. Operator interface module 478includes manual controls, including a multi-function, on/off, modecontrol switch and button 479, up and down data entry slewing buttons480, 481, and a digital display 482. Display 482 is controllable byswitch 479 to selectively display air pressure within and force onselectable air bladder cells 422, and the sum and average of all forcesexerted on sensors 433.

As shown in FIG. 42, electro-pneumatic controller 420A is contained in abox-like housing 483 which has protruding from a rear panel 484 thereofan L-bracket 485 for suspending the housing from a side board or endboard of a bed. Housing 483 of electro-pneumatic controller 420A alsoincludes a tubular member 486 for interfacing air hoses 487 with airbladder cells 422, row and column conductors 488, 489, to sensors 433 ofsensor array 432, and an electrical power cord 490 to a source ofelectrical power for powering the components of apparatus 420A.

Force Minimization Algorithm

The force minimization apparatus described above is made up of amultiplicity of air 424 bladder cells 422. Each cell 422 has on itsupper surface a separate force sensor 433. An air pressure transducer444 is provided to measure the air pressure in each cell. Each forcesensor is located in a potential contact region between a person lyingon cushion 421 and the air bladder cell. Each piezoresistive forcesensor 433 functions as a force sensitive transducer which has anelectrical resistance that is inversely proportional to the maximumforce exerted by a person's body on the air bladder cell 422, themaximum force corresponding to the lowest resistance path across anypart of each sensor.

As shown in FIG. 37, each air bladder cell 422 supports a differentlongitudinal zone of the user such as the head, hips or heels. Thecompressor 440 and selector valves 443 controlling the air pressure ineach zone are controlled by sensors 433 and pressure measurements madeby pressure transducer 444, using a novel algorithm implemented incomputer 437. There can be a minimum of one zone using one air bladdercell 433, and up to N zones using n air bladder cells, wherein each zonehas a force sensor 433 to measure the maximum force on that air bladdercell, the pressure transducer 444 being used to measure the air pressurein that air bladder cell. The control algorithm is one of continuousiteration wherein the force sensors 433 determine the peak force on thepatient's body, and the pressure transducer 444 measures the pressure atwhich the force occurs. At the end of a cycle sampling forces on allsensors, the bladder air pressure is restored to the pressure where theforce was minimized for all zones. This process continues and theapparatus constantly hunts to find the optimal bladder pressures foreach individual cell resulting in minimizing peak forces on a personsupported by overlay cushion 421.

Algorithm Description:

Given:

N Zones each containing one air bladder cell and numbered one to N

The air bladder cell of each zone is selectably connectable to an airpressure transducer to measure P#

Each air bladder cell is fitted with an individual force sensor capableof measuring the maximum force F# exerted on the surface of each cell.

A common compressor supplies air at pressures of up to 5 psi to selectedindividual air bladder cells of the zones. There is a normally closedvent valve for deflating a selected air bladder cell by exhausting airto the atmosphere through the vent valve.

There is a selector valve that selects which air bladder is beinginflated with air or deflated by exhausting air to the atmospherethrough the vent valve.

Algorithm Steps:

1. Pset:::: Pset, start, close vent valve

2. Select zone i=1 by opening selector valve 1

3. Turn the compressor on.

4. Measure the air pressure in the air bladder cell in zone I

5. Pressurize the zone-one air bladder cell to a predetermined upper setpressure and close the selector valve value Pset.

6. Repeat for i+1 until i+1=N

7. Select Zone i=I

8. Obtain the force sensor readings for all zones.

9. Open Vent valve.

10. Deflate the zone-one air bladder cell to a predetermined minimumpressure and monitor all the force sensor readings on all air bladdercells. Maintain bladder pressures in all other air bladder cells attheir upper set pressures. Measure forces on all air bladder cells asthe single, zone-one air bladder is being deflated and compute the sumand optionally the average of all force sensor readings.

12. Store in computer memory the pressure reading of the zone-one airbladder cell at which the minimum sum and optionally the average of allforce sensor readings occurs.

13. Restore the pressure in the zone one air bladder cell to the valuewhere the minimum sum and average force sensor readings for all theforce sensors was obtained.

14. Close the zone-one selector valve. Maintain the pressure in zoneone.

15. Set: Count=i+1.

16. Repeat steps 2 thru 15 until Count=i+1=N.

17. Set: Pset=Pset, start−(Count*20%_(i.e., reduce the initial pressurein the zone one bladder).

18. Repeat Steps 2 thru 16 (i.e., with a reduced initial pressure).

Caveat

19. Constantly monitor all force sensors and if significant change(Delta F>0.2*F#) is detected (patient moved) start over at Step 1.

FIG. 43 is a flow chart showing the operation of apparatus 420 utilizingthe algorithm described above. Table 1 lists appropriate lower and upperinitial set pressures for bladders 422, as a function of the weight of apatient or other person supported by overlay cushion 421 of theapparatus.

TABLE 1 Patient Weight Minimum Pressures Start Pressure  75-119 Pounds5.5″ ± 0.7:H₂O 6.5″ ± 0.7:H₂O 10.31 ± 2 mmHg 12.18 ± 2 mmHg 120-164Pounds 6″ ± 0.7:H₂O 8″ ± 0.7:H₂O 11.25 ± 2 mmHg 15 ± 2 mmHg 165-199Pounds 8″ ± 0.7:H₂O 10″ ± 0.7:H₂O 15 ± 2 mmHg 18.75 ± 2 mmHg 200-250Pounds 10″ ± 0.7:H₂0 12″ ± 0.7:H₂O 18.75 ± 2 mmHg 22.49 ± 2 mmHg MaximumPressure 26″ ± 0.7:H₂O 48.74 ± 4 mmHg

In a variation of the method and apparatus according described above,after the pressures in each air bladder cell 422 have been optimized forminimum force concentration, inlet tubes 431 may be permanently sealed,and the adaptive cushion 421 permanently disconnected from pressurecontrol module 475. This variation would also enable the customfabrication of cushions 421 using air bladder cells 422, for customizingchair cushions to minimize force concentrations on a particularindividual. Similarly, the variation of the method and apparatus couldbe used to customize saddle cushions or car seats.

What is claimed is:
 1. A flexible force sensing array comprising: an elastically stretchable sheet; a plurality of first conductive paths supported on said elastically stretchable sheet; a layer of sensing material positioned in contact with said first conductive paths, said layer of sensing material having an electrical characteristic that varies in response to physical forces exerted thereon; a layer of semiconductive material positioned in contact with said layer of sensing material on a side of said layer of sensing material opposite said plurality of first conductive paths; and a plurality of second conductive paths positioned in contact with said layer of semiconductive material on a side of said layer of semiconductive material opposite said layer of sensing material.
 2. The array of claim 1 wherein said layer of sensing material includes a layer of piezoresistive material.
 3. The array of claim 2 wherein said layer of piezoresistive material is supported by an elastically stretchable substrate.
 4. The array of claim 3 wherein said elastically stretchable substrate is made at least partially of nylon.
 5. The array of claim 4 wherein said plurality of second conductive paths are supported on a second elastically stretchable sheet, and said elastically stretchable sheet and said second elastically stretchable sheet are both made from woven fabric.
 6. The array of claim 1 wherein said plurality of second conductive paths are supported on a second elastically stretchable sheet.
 7. The array of claim 6 wherein said elastically stretchable sheet and said second elastically stretchable sheet are both made from woven fabric.
 8. The array of claim 8 wherein said woven fabric includes nylon.
 9. The array of claim 1 wherein said semiconductive layer is coated onto said layer of sensing material and includes a metallic oxide.
 10. The array of claim 2 wherein said semiconductive layer is coated onto said layer of sensing material and includes a metallic oxide.
 11. The array of claim 2 further including a cover sheet positioned in contact with said elastically stretchable sheet, said cover sheet being made of thin, flexible, and elastically stretchable material.
 12. A flexible force sensing array comprising: a first elastically stretchable sheet; a plurality of first conductive paths supported on said first elastically stretchable sheet, said first conductive paths being parallel to each other; an intermediate elastically stretchable sheet positioned in contact with said first elastically stretchable sheet, said intermediate elastically stretchable sheet including sensing material thereon that has an electrical characteristic that varies in response to applied physical forces; a layer of semiconductive material positioned in contact with said intermediate elastically stretchable sheet to thereby form with said sensing material a PN junction; a second elastically stretchable sheet; and a plurality of second conductive paths supported on said second elastically stretchable sheet and in electrical contact with said semiconductive material, said second conductive paths being parallel to each other and transverse to said first conductive paths.
 13. The array of claim 12 wherein said sensing material is a piezoresistive material.
 14. The array of claim 13 wherein said first and second elastically stretchable sheets are both made at least partially of nylon.
 15. The array of claim 14 wherein said semiconductive layer includes a metallic oxide.
 16. The array of claim 14 wherein semiconductive layer includes copper oxide.
 17. The array of claim 13 further including a cover sheet positioned in contact with said first elastically stretchable sheet, said cover sheet being made of thin, flexible, and elastically stretchable material.
 18. The array of claim 13 further including: a plurality of air bladder cells coupled to one side of said array; a controller in communication with said array; and a source of pressurized air in communication with said controller wherein said controller adjusts the inflation levels within said air bladder cells based upon outputs from said flexible force sensing array.
 19. The array of claim 18 wherein said controller is adapted to adjust the inflation levels within said air bladder cells such that interface forces exerted on a patient positioned on said air bladder cells are less concentrated. 